Extend verification of FSFS f7 repositories.

* subversion/libsvn_fs_fs/verify.c
  (compare_p2l_to_l2p_index): Empty regions only have P2L index entries
                              but we can still check their revision range
                              if given.
  (compare_p2l_to_rev): All non-empty sections do have valid checksums.
                        Verify them even if they are 0.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631197 13f79535-47bb-0310-9956-ffa450edef68